The present invention relates to an impressed current cathodic protection monitoring system for marine apparatus and particularly to equipment for monitoring an impressed current automatically controlled cathodic protection system for protecting underwater metal such as marine outboard motors, stern drive propulsion units and the like.
Underwater metallic structures of marine propulsion devices are subject to damaging corrosion particularly when the marine apparatus is employed in salt water and other water environments. Outboard motors and the stern drive component of an inboard-outboard marine propulsion unit are secured to the aft end or transom of the boat with a pendant propeller propulsion means which extends downwardly from the transom below the water line. Such propeller units are metal and generally consist of aluminum and steel. Generally, other metals will be associated with the lower unit. In such practical constructions, the lower submergible unit is highly subject to corrosive action as a result of the galvanic potential difference between the metal components. The problem is particularly severe in salt water environments. It is well-known that current produced by a sacrificial anode or direct current (D.C.) impressed on a permanent anode can be mounted to the transom below the water level to create a protective polarization of the lower unit (the cathode) to retard such corrosive action. The D.C. power sources must have the positive side coupled to the anode and the negative side coupled to the metal pendant portions to be protected from corrosion. The latter thus functions as the cathode with respect to the anode. By maintaining the anode at an appropriate potential, current is supplied to the cathode which maintains a protective polarization thereon which essentially prevents corrosion. The particular potential at which the anode is maintained is significant for optimum operation. Where the pendant unit is formed of aluminum, the protected metal member should be maintained at a negative potential of approximately 940 millivolts with respect to a silver-silver chloride reference electrode. In practical systems, a reference electrode is also mounted below the water line and coupled into a controller to maintain maximum effectiveness. To maintain this precise potential, the automatic controller varies the current impressed on the anode. The controller is generally a solid state regulating circuit employing a reference electrode such as a silver base coated with a silver chloride. Variations of the reference potential with respect to the metal member establishes a continuous signal to the controller to vary the driving potential of the anode current until the submerged metal pendant unit is at the desired polarization potential. Thereafter, the controller functions to maintain such an optimum polarization level. Such systems are well-known and are often employed in small, recreational type boats where they are subject to relatively severe physical conditions of bouncing and jarring. As a result, disruption of the circuit connection and the system may occur. Generally, the operation of the system may only be detected by noticing the unwarranted corrosion.
Although galvanometer type systems have been employed in the laboratory to monitor the operation of cathodic protection systems, such systems are completely unacceptable from a practical standpoint for use in marine propulsion devices. Such systems employ highly delicate instruments which cannot readily withstand the physical conditions encountered in marine propulsion units particularly small, recreational boats and they are also subject to corrosion. Further, galvanometer units are relatively expensive and would not, therefore, find wide acceptability. Consequently, the boating industry has relied on visual indications after the fact or special surveys using laboratory type instruments.